O,S-dialkyl phosphoramidothioates are effective insecticides. One particularly effective insecticide is O,S-dimethyl phosphoramidothioate (See, e.g., U.S. Pat. Nos. 3,309,266, 3,639,547 and 4,389,350, the disclosures of which are incorporated herein by reference). U.S. Pat. No. 3,309,266 discloses that O,S-dimethyl phosphoramidothioate can be made by reacting O,O-dimethyl chlorophosphorothioate with ammonia or a primary alkylamine and then heating the product of that reaction in the presence of an alkylating reagent such as an alkyl halide.
U.S. Pat. No. 3,639,547 discloses that O,S-dimethyl phosphoramidothioate can be made by reacting O,O-dimethyl phosphoramidothioate with the dimethyl ester of sulfuric acid or with a methyl ester of organic sulfonic acids. The reaction occurs at a temperature of from about 20.degree. C. to about 100.degree. C. In a manner similar to the method disclosed in U.S. Pat. No. 3,309,266, the O,O-dimethyl phosphoramidothioate can be made via ammoniation of a O,O-dimethyl halophosphorothioate.
With either of the above methods, the yield and purity of the formed O,S-dimethyl phosphoramidothioate are low, ranging from about 30 to 75 percent.
U.S. Pat. No. 4,389,350 describes a process for increasing the yield of the O,S-dimethyl phosphoramidothioate. The process involves i) heating the O,O-dimethyl starting material in the presence of a catalyst to a temperature of from 40.degree. C. to 70.degree. C., ii) arresting the heating when about 40 to 70% of the O,O-dimethyl starting material has isomerized to O,S-dialkyl product, iii) separating unreacted O,O-dialkyl starting material from the O,S-dialkyl isomerization product and iv) further treating the O,O-dialkyl starting material to effect further isomerization at a temperature of from 40.degree. C. to 70.degree. C. While slightly better yields are reported, the process involved is relatively complicated.
There continues to be a need in the art, therefore for a simple, efficient method for making O,S-dimethyl phosphoramidothioate.